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Deep Drawing Steels

Deep drawing steels are used normally in the form of sheets which are formed into formed products by a sheet forming process. The commonly used processes for sheet forming are deep drawing and stretch forming processes. Deep drawingis the most popular steel forming process available to manufacturers since it offers outstanding forming characteristics and good resistance to ageing. Unlike steel stamping, which typically punches out a part, deep drawing is a process that involves using a die to form blank sheets of metal into a desired shape around a punch. Deep drawn parts are distinguished by a depth equal to more than half of the diameter of the part. Numerous re-draws can be used to attain greater depths. The wall thickness in case of deep drawn products is nearly the same as the thickness of the sheet blanks as compared to stretch forming where the thickness decreases. This manufacturing process has the advantages of unit cost, weight savings and flexibility with design over other traditional processes. Deep drawing steel can be hot rolled or cold rolled or can be coated steels such as galvanized iron or galvannealed steel sheet. However the surface structures and excellent formability properties of cold rolled steel grades make them ideal for the purpose of deep drawing.

Deep drawing steels are used extensively in automobile industry. The range of deep drawing steels covers four levels based on the difficulties faced during forming operations. These four levels include drawing, deep drawing, extra deep drawing and extra deep drawing plus. The range of deep drawing steels include non alloyed mild steels, dual phase steels, rephosphorized steels, interstitial free steels with very limited carbon content, other advanced high strength steels and stainless steels.

During the process of deep drawing a high level of rejection takes place in case the design of die is not proper or if the steel does not have the properties needed for deep drawing. For deep drawing, steel must have good formability which means the steel has the ability of plastic deformation without fracture and loss of stability. The basic parameters which influence the steel property of deep drawing are as follows:

Production methods mainly related to secondary steel making and rolling processes

Chemical composition

Micro structure

Mechanical properties

Surface quality

Dimensional tolerances

Vertical anisotropy

The property of formability in steels is a result of the interaction of many variables, the main ones being the mechanical properties of the steel, the forming system (tooling) used to manufacture parts, and the lubrication used during forming. Tight control over chemical composition, hot rolling parameters, amount of cold reduction, annealing time and temperature, and the amount of temper rolling allow the production of steels with good formability. To prevent the occurrence of fluting or stretcher strains during forming, deep drawing steels are tempered as a normal step in the mill processing.

Indicative information of deep drawing properties of steels can be obtained from their behaviour during the tensile test. For the pressability criterion of steel, the properties of the yield point, the tensile strength, the strain uniform deformation and the work hardening exponent are used. Also by ensuring the low scatter in the mechanical properties in the deep drawing steels, the optimum productivity in the drawing press operation is achieved.

The pressability of steel is evaluated using technological test such as Erichsen cupping test. The deep drawing steels should have extremely low carbon content. These steels can be either alloyed with special alloying elements or can be unalloyed steels with special rolling strategies in order to meet demands for lowest possible yield strength levels, good cold formability properties and good ageing resistance. Deep drawing steels are usually specified by yield strength and tensile strength and by minimum elongation values. For forming properties, minimum values for vertical anisotropy (r value) and work hardening exponent (n value) are normally specified.

Deep drawing steels have a pure ferritic microstructure or consist of a ferritic matrix which may contain isolated grainy carbides. The grain boundaries and fine carbides in these steels are visible if the steel is etched with Nital.

Erichsen cupping test

The Erichsen cupping test is basically a ductility test which is employed to evaluate the ability of steel sheets to undergo plastic deformation during the forming operations. This is the one of the best deep drawing test method for steel sheet and was world-wide patented as early as 1913 by Mr. Erichsen. To conduct this test, a steel sheet specimen is clamped between a blank holder and a die and then dented (deep drawn) with a hardened spherical punch. This process is continued at a prescribed speed until it results in a fine, continuous crack in the steel sheet. The displacement of the spherical punch till cracking occurs is known as the Erichsen cupping index (IE).

Another test which is carried out is the deep drawing cup test for finding out the limiting drawing ratio which is a quality attribute for the forming ability of the steel sheet material. For this test a circular plate (blank) is stamped from the steel sheet to be tested and then formed into a cup using a drawing die and a drawing punch. The greatest possible ratio between the blank and the drawing punch diameter, which just permits the faultless production of a cup is called limiting drawing ratio. The ears, which form as a result of the flow properties of the material, are undesirable because they necessitate rework on the drawn products when they occur in practice.

The principle of the Erichsen cup test and the deep drawing cup test is shown in Fig 1.

Deep drawing steels are suitable for several types of forming operations. They are used both for visible and structural parts of an automobile. Along with very high elongation values and a wide range of yield strengths, these steels have outstanding forming properties. The choice of steel grade for a particular forming operation must take into account the anticipated forming stresses, so that the individual advantages of the grades can be optimally exploited and the steel can be used for difficult drawing operation. In deep drawing operations,, measured in terms of the limiting draw ratio, grades with greater resistance to thinning i.e. a high r value perform better. In a combination of stretch forming and deep drawing, steels with equally high r value and n value are advantageous. Further though deep drawing steels are specified in terms of their formability, they ensure a minimum performance as regards crash behaviour and fatigue strength and represent the lower limit with regard to loadability (strength, crash resistance and fatigue strength).

The most frequently used joining methods for deep drawing steels are resistance and shield gas welding. The preferred resistance welding processes are spot welding, projection welding and roll seam welding